extract LibraryPrefixContributor from imported reference contributor

pkg/analysis_server/test/services/completion/dart/test_all.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library test.services.completion.dart;
 
 import 'package:unittest/unittest.dart';
 
 import '../../../utils.dart';
 import 'arglist_contributor_test.dart' as arglist_test;
 import 'combinator_contributor_test.dart' as combinator_test;
 import 'common_usage_sorter_test.dart' as common_usage_test;
 import 'field_formal_contributor_test.dart' as field_formal_contributor_test;
 import 'inherited_contributor_test.dart' as inherited_contributor_test;
 import 'keyword_contributor_test.dart' as keyword_test;
-import 'library_member_contributor_test.dart' as library_test;
+import 'library_member_contributor_test.dart' as library_member_test;
+import 'library_prefix_contributor_test.dart' as library_prefix_test;
 import 'named_constructor_contributor_test.dart' as named_contributor_test;
 import 'static_member_contributor_test.dart' as static_contributor_test;
 import 'type_member_contributor_test.dart' as type_member_contributor_test;
 import 'uri_contributor_test.dart' as uri_contributor_test;
 
 /// Utility for manually running all tests.
 main() {
   initializeTestEnvironment();
   group('dart/completion', () {
     arglist_test.main();
     combinator_test.main();
     common_usage_test.main();
     field_formal_contributor_test.main();
     inherited_contributor_test.main();
     keyword_test.main();
-    library_test.main();
+    library_member_test.main();
+    library_prefix_test.main();
     named_contributor_test.main();
     static_contributor_test.main();
     type_member_contributor_test.main();
     uri_contributor_test.main();
   });
 }